Electrodynamic drives are used in respiration technology to actuate valves. The valves may be gas dispensing valves, which make the breathing gas flow available, or valves within a respiration system for controlling phases of inspiration and expiration. Electrodynamic drives are part of a position control circuit, with which a certain opening stroke of the valve must be set in the shortest time possible. A velocity-proportional damping is needed for this, which is sent in the inverted form to the position control circuit.
Electrodynamic drive systems are known, in which a velocity-measuring coil is wound coaxially over the drive coil. This system has the advantage of being of a very compact design, because no additional space is needed for installation. The space requirement for the additional winding is negligible. The drawback of this solution is the electrical coupling between the drive coil and the measuring coil. A change in current in the drive coil induces a voltage in the velocity-measuring coil according to the transformer principle, i.e., not only is a velocity-proportional voltage generated, which is needed for the damping, but a change in the driving current is reproduced as well. This effect cannot be eliminated by a compensation by calculation, because this would have to take place very rapidly, in the range of about 200 Msec., and, moreover, great specimen dispersions are to be taken into account as well. The compensation could therefore take place, for reasons of stability, to a very low percentage only. The consequence of this is a damping set at a relatively low value. Damping set too high causes the system to be damped too greatly initially in case of dynamic changes and an acceleration to be too slow in case of a change in the command variable.
If, by contrast, there is a pneumatic disturbance variable in the frequency range of 50 Hz to 500 Hz, especially the higher interfering frequencies cannot be damped sufficiently strongly, because the phase shift between the real velocity and the apparent velocity generated by the transformatory coupling may be 180°. If an intensified negative feedback were offered to this apparent velocity, a positive feedback would be obtained instead of damping, and this positive feedback may generate a markedly perceptible continuous oscillation.
An electrodynamic drive in which the velocity-measuring coil is arranged coaxially over the driving direction appears, for example, from U.S. Pat. No. 7,030,519 (incorporated herein by reference).
Another system, which represents the state of the art, is a drive with a flanged velocity-measuring system. The drawbacks of the electrical coupling can be extensively avoided with this design, but this advantage is obtained at the expense of the drawback of a markedly greater space requirement, which greatly limits the possibility of installation under crowded conditions, as they frequently occur in modern, compact devices. In addition, external velocity-measuring systems are associated with the design problem of coupling with the driving coil. If a minimum clearance is left between the velocity measurement and the moving mass of the drive system, a two-mass oscillator is obtained, which may generate parasitic oscillations due to the phase shift in case of a strong negative feedback of the velocity signal.
An electrodynamic drive with flanged velocity measuring system is known from U.S. Pat. No. 5,127,400 (incorporated herein by reference).